This invention relates to a vapor generator, and more particularly, to a sub-critical or super-critical once-through generator operating at variable pressure for converting water to steam.
The trend in the generation of electrical power is to the use of large fossil fired vapor generators to maintain the cyclic or peaking load requirements of a utility's distribution system. This type of use dictates that these type generators be capable of rapid load changes on line. For example, a utility distribution system may require from the steam generator as much as 3% per minute load change capability, and in some instances a load change rate as high as 5% to 10% per minute. Also the large steam generators must be able to achieve rapid warm starts following overnight or weekend shutdown.
It is well recognized that rapid load changes of the above type from the point of view of turbine life can best be accomplished by operating the vapor generator at variable pressure since variable throttle pressure operation with full arc admission for the turbine produces minimal change to the first stage temperature and therefore can accommodate rapid load changes with minimum turbine rotor damage.
An additional advantage of variable pressure operation is that during cold and hot starts for the vapor generator, close matching of fluid temperature to turbine metal temperature is more easily achieved to limit fatigue damage to turbine inlet parts.
However, a once-through vapor generator designed for variable pressure operation must maintain satisfactory characteristics of flow in the furnace circuitry and minimize circuit flow unbalance caused by heat upset or uneven distribution of steam and water to a circuit.
U.S. Pat. No. 3,789,806 issued on Feb. 5, 1978, to the present applicant and assigned to the same assignee as the present invention, teaches the use of a once-through vapor generator capable of variable pressure operation in which the functional performance of the furnace circuitry at different loads and pressures was satisfactory to overcome the above disadvantages. In this arrangement, and particularly the arrangement disclosed in the embodiment of FIGS. 9 and 10, initial flow of fluid through the furnace circuitry was through the lower portions of the furnace boundary sidewalls. The fluid was then passed to mix headers and then simultaneously through the front wall, the rear wall and the sidewall end panels before passing through the upper sidewall portions. However, in this arrangement, the horizontal span of the lower sidewall portions through which the fluid was initially passed was large and to limit the enthalpy pick-up of this circuit an intermediate mix header was used. The use of the mix header is undesirable from a sealing and maintenance aspect, and is costly.